A variety of different methods are known for fabricating Group III/Group V semiconductor devices. For example, it is well known in the art to fabricate doped single crystal epitaxial layers in the InGaAsP materials system on an InP substrate using conventional growth techniques such as Liquid Phase Epitaxy (LPE), Molecular Beam Epitaxy (MBE), Metal-Organic Chemical Vapor Deposition (MOCVD), and Metal-Organic Vapor Phase Epitaxy (MOVPE).
It is well known in the art to fashion such Group III/Group V layers into an assortment of optoelectronic semiconductor devices, such as semiconductor lasers and LEDs. Generally, these optoelectronic devices are double heterostructures having an active layer sandwiched between a buffer layer and a cladding layer. Frequently, these optoelectronic devices also include a cap layer on top of the cladding layer so that an ohmic contact with a subsequently deposited metal contact layer is formed.
Despite zinc's wide use as a p-type dopant in InP-InGaAsP semiconductor devices, such as in the active layers of InP-InGaAsP double heterostructures, the exact mechanism of the incorporation of zinc into the InGaAsP crystal lattice so that it acts as an acceptor is not well understood.
As used herein a material in the InGaAsP materials system refers to a semiconductor alloy having a composition In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y, within the range 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1.
A number of researchers have recently considered how zinc may be incorporated as an acceptor in the InGaAsP materials system. For example, the acceptor concentration (i.e. the electrical doping level) of an MOVPE grown p-type InGaAsP capping layer cooled from typical MOVPE growth temperatures is strongly dependent on the gaseous cooling ambient. This is the result of electrical deactivation rather than loss of dopant. Specifically, when the cooling ambient is PH.sub.3 or AsH.sub.3, atomic hydrogen is bound in the crystal lattice and serves to deactivate the Zn (see e.g.; S. Cole et al "Effect of Cooling Ambient on Electrical Activation of Dopants in MOVPE of InP" Electronics Letters, Jul. 21, 1988, Vol 24, No. 15, pp. 929-931; and G. R. Antell et al "Passivation of Zinc Acceptors in InP by Atomic Hydrogen Coming from Arsine During Metalorganic Vapor Phase Epitaxy," Appl Phys Lett 53(9) Aug. 29, 1988 pp. 758-760). In particular, it has been observed that the acceptor concentration of a sample cooled in AsH.sub.3 can be restored by heat treating in PH.sub.3 or H.sub.2 for a short period of time (see European Patent application 0 242 084).
In a recent series of papers, Van Gurp et al have shown that closed ampoule Zn diffusion in an InP surface layer results in a net acceptor concentration that is much smaller than the Zn concentration. In particular, Van Gurp et al have shown that a relatively large fraction of the total zinc atoms diffused into the InP are not incorporated into the crystal lattice but instead are located in interstitial sites. The interstitial zinc atoms act as a donor species rather than as an acceptor species and offset the acceptor properties of the substitutional zinc atoms. This results in a net acceptor concentration that is much smaller than the total zinc concentration. Van Gurp et al have also shown that when a zinc diffused surface layer is subsequently annealed at a temperature in the range of 475.degree.-500.degree. C., the interstitial zinc can redistribute, giving rise to a substantial increase in the net acceptor concentration. Van Gurp et al explain their results as occurring because zinc is incorporated as both substitutional acceptors and interstitial donors in the InP surface layer; however, when the InP surface layer is annealed, only the interstitial zinc is driven out via the surface owing to its large diffusion coefficient. Thus, the ratio of substitutional zinc to interstitial zinc increases giving rise to an apparent increase in the net acceptor concentration. (See, Van Gurp et al, 65 J. Appl. Phys. 553 (1989), 64 J. Appl. Phys. 3468 (1988), and 61 J. Appl. Phys. 1846 (1987)).
G. Dlubek et al (see "Vacancy--Zn Complexes Studied by Positrons" Appl. Phys. Lett 46(12) Jun. 15, 1985; pp 1136-1138) also discuss the formation of Zn complexes's in Zn doped InP bulk crystals and the disassociation of such complexes and out diffusion of Zn from the surface via the evaporation of interstitial zinc at temperatures above 400.degree. C.
In short, the above described prior art indicates that the mechanism by which zinc is incorporated into the lattice of an InP/InGaAsP material and activated to act as acceptor is not well understood.
In particular, the prior art provides no generally applicable technique for controlling the activation of zinc as an acceptor in the active layer of a Group III/Group IV semiconductor device. The technique for activating zinc described in the above mentioned European Patent Application relates only to the situation where a MOVPE grown layer is initially cooled in an atmosphere containing AsH.sub.3 so that hydrogen is incorporated in the crystal lattice to passivate zinc and wherein the layer is reheated in an atmosphere of PH.sub.3 or H.sub.2 to reactivate the zinc.
This technique has a number of shortcomings. Firstly, it can only be used to activate zinc in a MOVPE grown layer initially cooled in an AsH.sub.3 ambient. In addition, it has not been shown to be applicable to the active layer of a device but only to an upper cladding layer and a cap layer in a double heterostructure. Thirdly, the technique is not applicable for activating zinc in LPE grown layer such as the active layer of an LPE grown double heterostructure.
The Van Gurp et al and Dlubek et al references only describe the out diffusion of interstitial zinc from the surface of an InP material to increase the net zinc acceptor concentration. The technique of Van Gurp and Dlubek therefore also has a number of shortcomings. In particular, it has not been shown in the above-mentioned references that the technique can activate zinc as an acceptor in the active layer of a device. The technique is especially not applicable for activating zinc in the active layer of a double heterostructure device wherein the active layer is not a surface layer but is sandwiched between a cladding layer and a buffer layer. The technique of Van Gurp and Dlubek is especially not applicable to activating zinc in the active layer of an LPE grown double heterostructure device.
In addition, none of the prior art in zinc activation techniques disclose how to control the activation of zinc as an acceptor in an active layer to improve the power output of an LED or laser such as a double heterostructure LED or laser.
Accordingly, it is an object of the present invention to provide a method for activating zinc impurities in the active layer of a Group III/Group V semiconductor device.
It is a further object of the invention to provide a method for activating zinc impurities in the active layer of a double heterostructure device such as an InP/InGaAsP double heterostructure LED or laser.
It is a further object of the invention to provide a method for activating zinc as an acceptor in an LPE grown Group III/Group V layer such as a layer formed in the InGaAsP materials system.
It is a further object of the invention to provide a method for activating zinc as an acceptor in the active layer of a light emitting semiconductor device such as an InP/InGaAsP double heterostructure LED or laser to improve the power output characteristics.